The present invention pertains to a pressure control device for the workpiece transport mechanism in sewing machines in which said mechanism is of the type that is adapted to maintain contact with the workpiece during the performance of its intended function. Additionally the invention relates to a means for increasing or decreasing the amount of pressure with which the transport mechanism engages a workpiece to effect its advancement through the sewing area of the sewing machine.
As is well known to those conversant in the art, the transport mechanisms of sewing machines are disposed and function in a manner so as to influence a particular portion of a workpiece, that generally consists of super-posed layers of fabric, so as to effect its advance through the sewing zone in synchronization with other active elements of said sewing machine operatively associated therewith. More particularly, the transport mechanisms are formed by elements including the well known feed dog or such elements as rotatably driven wheels or discs that are caused to act on a workpiece intermittently and in a direction which will permit the desired formation of stitches in said workpiece.
It is also well known that these transport mechanisms whether they be of the feed dog or disc type, can be caused to influence only one of the layers of fabric of a workpiece or they can be utilized in combination with the feed dog type being effective on the lower layer of fabric and the disc type on the upper layer.
Generally, more than one transport mechanism is provided on sewing machines which are utilized to assemble workpieces that define articles of clothing so that when advancing or transporting said workpiece through the sewing zone, the various layers of material comprising such articles will be maintained in their intended alignment with one another.
When utilizing more than one transport mechanism, the lower layer of the workpiece is acted upon the mechanism which is caused to periodically project above the work surface of the machine and cooperate with the machine's presserfoot to effect advance of said workpiece. The transport mechanism of the upper layer being generally of the disc type engages its respective layer at a position lower than the presser foot and performs its function in opposition to the machine's work surface that is fixedly positioned.
Regardless of which type of transport mechanism that is utilized, the pressure which it exerts on the upper layer to effect its advance tends to compress it and the forces of this compression becomes greater in the event of an increase in thickness and softness of said upper layer.
When the forces of compression by the transport mechanism become what is considered excessive, they frequently create an undesireable condition of marking the workpiece with the mark extending the entire distance along that portion of the workpiece which was acted upon by said mechanism.
With workpieces formed from particularly soft fabrics such as cord or velvet, the amount of compression by the transport mechanism becomes appreciable relative to the thickness of the workpiece and the amount by which the upper layer is advancd with respect to the lower layer is caused to be increased and the mark produced by the transport mechanism becomes more pronounced and in many instances results in a permanent mark which cannot be removed. This increase in the amount the upper layer is caused to advance relative to the lower layer, can be attributed to the deeper penetration of the transport mechanism into the upper layer which alters both the degree of friction between the two layers that are in contact with one another, and the effective amplitude of the advancing motion of the transport mechanism. For practical and economic reasons it is not possible to continuously adjust the rate of advance of a workpiece and the pressure exerted by the transport mechanism on the upper layer in order to adapt the latter to the thickness and the particular type of fabric forming the workpiece which is being processed.
In order to alleviate the above described disadvantages devices are known for selectively adjusting the height of the transport mechanism so that the pressure with which they act on a workpiece can be controlled within satisfactory limits. Such known devices usually consist of a regulating screw which is arranged to oppose the action of the biasing means of the transport mechanism and it is quite obvious that such devices would be inconvenient as well as impractical when a series of workpieces being sewn vary in thickness from one to another.
An object of the present invention is to provide a device for regulating the pressure with which the transport mechanism engages a workpiece without altering the intended function of said mechanism and which can be quickly and easily adjusted to satisfactorily accommodate a series of workpieces which have thicknesses that vary from one to another.